This invention relates to controlling interwinding coupling coefficients and leakage inductances of a transformer, and use of such a transformer in a high-frequency switching circuit, such as, for example, a high frequency switching power converter.
With reference to FIG. 1, which shows a schematic representation of an electronic transformer having two windings 12, 14, the lines of flux associated with current flow in the windings will close upon themselves along a variety of paths. Some of the flux will link both windings (e.g. flux lines 16), and some will not (e.g. flux lines 20, 22, 23, 24, 26). Flux which links both windings is referred to as mutual flux; flux which links only one winding is referred to as leakage flux. The extent to which flux generated in one winding also links the other winding is expressed in terms of the winding""s coupling coefficient: a coupling coefficient of unity implies perfect coupling (i.e. all of the flux which links that winding also links the other winding) and an absence of leakage flux (i.e. none of the flux which links that winding links that winding alone). From a circuit viewpoint, the effects of leakage flux are accounted for by associating an equivalent lumped value of leakage inductance with each winding. An increase in the coupling coefficient translates into a reduction in leakage inductance: as the coupling coefficient approaches unity, the leakage inductance of the winding approaches zero.
Control of leakage inductance is of importance in switching power converters, which effect transfer of power from a source to a load, via the medium of a transformer, by means of the opening and closing of one or more switching elements connected to the transformer""s windings. Examples of switching power converters include DCxe2x80x94DC converters, switching amplifiers and cycloconverters. For example, in conventional pulse width modulated (PWM) converters, in which current in a transformer winding is interrupted by the opening and closing of one or more switching elements, and in which some or all of the energy stored in the leakage inductances is dissipated as switching losses in the switching elements, a low-leakage-inductance transformer (i.e. one in which efforts are made to reduce the leakage inductances to values which approach zero) is desired. For zero-current switching converters, in which a controlled amount of transformer leakage inductance forms part of the power train and governs various converter operating parameters (e.g. the value of characteristic time constant, the maximum output power rating of the converter; see, for example, Vinciarelli, U.S. Pat. No. 4,415,959, incorporated herein by reference), a controlled-leakage-inductance transformer (i.e. one which exhibits finite, controlled values of leakage inductance) is required. One trend in switching power conversion has been toward higher switching frequencies (i.e. the rate at which the switching elements included in a switching power converter are opened and closed). As switching frequency is increased (e.g. from 50 KHz to above 100 KHz) lower values of transformer leakage inductances are usually required to retain or improve converter performance. For example, if the transformer leakage inductances in a conventional PWM converter are fixed, then an increase in switching frequency will result in increased switching losses and an undesirable reduction in conversion efficiency (i.e. the fraction of the power drawn from the input source which is delivered to the load).
A transformer with widely separated windings has low interwinding (parasitic) capacitance, high static isolation, and is relatively simple to construct. In a conventional transformer, however, the coupling coefficients of the windings will decrease, and the leakage inductance will increase, as the windings are spaced further apart. If, for example, a transformer is configured as shown in FIG. 1, then flux line 23, generated by winding #1, will not link winding #2 and will therefore form part of the leakage field of winding #1. If, however, winding #2 were brought closer to, or overlapped, winding #1, then flux line 23 would form part of the mutual flux linking winding #2 and this would result in an increase in the coupling coefficient and a decrease in leakage inductance. Thus, in a transformer of the kind shown in FIG. 1, the coupling coefficients and leakage inductances depend upon the spatial relationship between the windings.
Prior art techniques for controlling leakage inductance have focused on arranging the spatial relationship between windings. Maximizing coupling between windings has been achieved by physically overlapping the windings, and a variety of construction techniques (e.g. segmentation and interleaving of windings) have been described for optimizing coupling and reducing undesirable side effects (e.g. proximity effects) associated with proximate windings. In other prior art schemes, multifilar or coaxial windings have been utilized which encourage leakage flux cancellation as a consequence of the spatial relationships which exist between current carrying members which form the windings, or both the magnetic medium and the windings are formed out of a plurality of small interconnected assemblies, as in xe2x80x9cmatrixxe2x80x9d transformers. Transformers utilizing multifilar or coaxial windings, or of matrix construction, exhibit essentially the same drawbacks as those using overlapping windings, but are even more difficult and complex to construct, especially where turns ratios other than unity are desired. Thus, prior art techniques for controlling coupling, which focus on proximity and construction of windings, sacrifice the benefits of winding separation.
It is well known that conductive shields can attenuate and alter the spatial distribution of a magnetic field. By appearing as a xe2x80x9cshorted turnxe2x80x9d to the component of time-varying magnetic flux which might otherwise impinge orthogonally to its surface, a conductive shield will support induced currents which will act to counteract the impinging field. Use of conductive shields around the outside of inductors and transformers is routinely used to minimize stray fields which might otherwise couple into nearby electrical assemblies. See, for example, Crepaz, Cerrino and Sommaruga, xe2x80x9cThe Reduction of the External Electromagnetic Field Produced by Reactors and Inductors for Power Electronicsxe2x80x9d, ICEM, 1986. Use of an electric conductor and a cylindrical conducting ring as a means of reducing leakage fields in induction heaters are described, respectively, in Takeda, U.S. Pat. No. 4,145,591, and Miyoshi and Omori, xe2x80x9cReduction of Magnetic Flux Leakage From an Induction Heating Rangexe2x80x9d, IEEE Transactions on Industry Applications, Vol 1A-19, No. 4, July/August 1983. British Patent Specification 990,418, published Apr. 28, 1965, illustrates how conductive shields, which form a partial turn around both the core and the windings of a transformer having tapewound windings, can be used to modify the distribution of the leakage field near the edges of the tapewound windings, thereby reducing losses caused by interaction of the leakage field with the current in the windings. Persson, U.S. Pat. No. 4,259,654, achieves a similar result by extending the width of the turn of a tapewound winding which is closest to the magnetic core.
The effects of conductive shields on the distribution of electric fields is also well known. In transformers, conductive sheets have been used as xe2x80x9cFaraday shieldsxe2x80x9d to reduce electrostatic coupling (i.e. capacitive coupling) between primary and secondary windings.
In embodiments of the invention, enhanced coupling coefficients and reduced leakage inductances of the windings of a transformer can be achieved while at the same time spacing the windings apart along the core (e.g. along a magnetic medium that defines flux paths) to assure safe isolation of the windings and to reduce the cost and complexity of manufacturing. Such transformers are especially useful in high frequency switching power converters where cost of manufacture must be minimized and where leakage inductances must either be kept very low, or set at controlled low values, so as to maintain high levels of conversion efficiency or govern certain converter operating parameters. These advantages are achieved by providing an electrically conductive medium, in the vicinity of the magnetic medium and windings, which defines a boundary within which emanation of flux from the magnetic medium and windings is confined and suppressed. The electrically conductive medium confines and suppresses the leakage flux as a result of eddy currents induced in the electrically conductive medium by the leakage flux. By appropriately configuring the electrically conductive medium, the spatial distribution of the leakage flux can be controlled to achieve a variety of benefits.
Thus, in general, in one aspect, the invention features a high frequency circuit having a transformer. The transformer includes an electromagnetic coupler having a magnetic medium providing flux paths within the medium, two or more windings enclosing the flux paths at separated locations along the flux paths, and an electrically conductive medium arranged in the vicinity of the electromagnetic coupler. The electrically conductive medium defines a boundary within which flux emanating from the electromagnetic coupler is confined and suppressed. The conductive medium thereby reduces the leakage inductance of one or more of the windings by at least 25%. Circuitry is connected to one or more of the windings to cause current in one or more of the windings to vary at an operating frequency above 100 KHz.
Preferred embodiments of the invention include the following features. For use as a switching power converter, the circuitry includes one or more switching elements connected to the windings, and the operating frequency is the switching frequency of the switching power converter. The electrically conductive medium is configured to reduce the leakage inductances of one or more of the windings by at least 75% at the operating frequency. In some embodiments, the electrically conductive medium is configured to restrict the emanation of flux from selected locations along the flux paths other than the locations at which the windings are located. In other embodiments, the electrically conductive medium is configured also to restrict the emanation of flux from the magnetic medium at selected locations along the flux paths which are enclosed by the windings.
In some embodiments, some or all of the electrically conductive medium comprises electrically conductive material formed over the surface of the magnetic medium. In some embodiments, some or all of the electrically conductive medium comprises electrically conductive material arranged in the vicinity of the electromagnetic coupler in the environment outside of the magnetic medium and the windings.
The conductive medium is configured to define a preselected spatial distribution of flux outside of the magnetic medium, and is arranged to preclude forming a shorted turn with respect to flux which couples the windings. Some or all of the conductive medium may comprise sheet metal formed to lie on a surface of the magnetic medium, or may be plated on the surface of the magnetic medium, or may be metal foil wound over the surface of the magnetic medium. Some or all of the conductive medium may be comprised of two or more layers of conductive materials. Some or all of the conductive medium may comprise copper or silver, or a superconductor, or a layer of silver plated over a layer of copper.
The conductive medium may include apertures which control the spatial distribution of leakage flux which passes between the apertures. The reluctance of the path, or paths, between the apertures may be reduced by interposing a magnetic medium along a portion of the path, or paths, between the apertures. A second electrically conductive medium may enclose some or all of the region between the apertures, the second conductive medium acting to confine the flux to the region enclosed by the second conductive medium. The second conductive medium may form a hollow tube which connects a pair of the apertures, the hollow tube being arranged to preclude forming a shorted turn with respect to flux passing between the apertures.
The conductive medium may comprise one or more conductive metal patterns arranged over the surface of the magnetic medium at locations along the flux paths. The conductive medium may enshroud essentially all of the surface of the magnetic medium at each of several distinct locations along the flux paths, or may enshroud essentially the entire surface of the magnetic medium.
The conductive medium may comprise one or more electrically conductive sheets arranged in the vicinity of the electromagnetic coupler in the environment outside of the magnetic medium and the windings. The windings and the magnetic medium lie in a first plane and the metallic sheets lie in planes parallel to the first plane. The metallic sheets form one or more of the surfaces of a switching power converter which includes the high frequency circuit. In some embodiments, the conductive medium comprises a hollow open-ended metallic tube arranged outside of the electromagnetic coupler. The thickness of the conductive medium may be one or more skin depths (or three or more skin depths) at the operating frequency. The domain of the magnetic medium is either singly, doubly, or multiply connected. One or more of the flux paths includes one or more gaps. The magnetic medium is formed by combining two or more (e.g., U-shaped) magnetic core pieces. The core pieces may have different values of magnetic permeability. One or more of the windings comprise one or more wires (or conductive tape) wound around the flux paths (e.g., over the surface of a hollow bobbin, each bobbin enclosing a segment of the magnetic medium along the flux paths).
In some embodiments, at least one of the windings comprises conductive runs formed on a substrate to serve as one portion of the winding, and conductors connected to the conductive runs to serve as another portion of the winding, the conductors and the conductive runs being electrically connected to form the winding. At least one of the conductors is connected to at least two of the conductive runs. The substrate comprises a printed circuit board and the runs are formed on the surface of the board. The magnetic medium comprises a magnetic core structure which is enclosed by the windings. The magnetic core structure forms magnetic flux paths lying in a plane parallel to the surface of the substrate.
In some embodiments, the conductive medium comprises electrically conductive metallic cups, each of the cups fitting snugly over the closed ends of the core pieces. Electrically conductive bands may be configured to cover essentially all of the surface of the magnetic domain at locations which are not covered by the first conductive medium, the bands being configured to preclude forming a shorted turn with respect to flux which couples the windings, the bands also being configured to restrict the emanation of flux from the surfaces which are covered by the bands at the operating frequency.
In general, in other aspects, the invention features the transformer itself, a switching power converter, a switching power converter module, and methods of controlling or minimizing leakage inductance, minimizing switching losses in switching power converters, transforming power, and making lot-of-one transformers.
Other advantages and features will become apparent from the following description and from the claims.